The invention relates generally to active clearance control for turbomachinery, and more particularly to clearance estimation for turbine engines.
Active clearance control systems for turbine engines may employ sensors to monitor the distance between two objects. By way of background, a turbine has a number of turbine blades that are disposed adjacent to a shroud. The clearance between each of the turbine blades and the shroud varies depending on the temperature of the respective turbine blade and shroud. For example, the clearance between the shroud and the turbine blades is greatest when the turbine is cold and gradually decreases as the turbine temperature increases. It is desirable that a minimal gap or clearance between the turbine blades and the shroud be maintained for effective operation of the turbine. A sensor may be disposed within the turbine to measure the distance between the respective turbine blades and the shroud. The distance may be used to direct movement of the shroud to maintain the desired displacement between the shroud and the turbine blades.
In certain applications, capacitive probes may be employed to measure the clearance between two objects. The probe is typically located on one of the objects and measures a capacitance with respect to the other object for estimating the clearance between the two objects. Unfortunately, existing measurement techniques employing capacitive probes may be inaccurate across the measurement range due to limitations on the signal-to-noise ratio (SNR) of such probes.
In certain systems, complex detection techniques are employed to enhance the SNR of the probe. For example, amplifiers are coupled to the sensors to amplify signals from the probe. Further, amplified signals are transmitted to a processing unit through cables for estimation of the clearance. However, such signals may include associated noise components as the signals travel from the amplifier to the processing unit. Furthermore, it is desirable to perform the amplification process as close as possible to the sensor, this requires that the amplifiers be designed to survive harsh environments such as within the turbine thereby resulting in reliability and lifetime limitations.
Moreover, in certain systems, large sensor elements are employed to increase the signal SNR. However, such probes are typically limited to large target geometries due to poor spatial resolution for small geometries. Certain systems employ resonant circuits to increase a gain of such probes. Unfortunately, high complexity calibration techniques may be required for such systems for addressing the resonance characteristics drifts of such circuits.
Accordingly, a need exists for providing a clearance estimation system that provides an accurate measurement of clearance between two objects by minimizing the effect of noise in a system.